In summary, cannulation and cardiopulmonary bypass is instituted by placing aortic pursestrings in the distal ascending aorta followed by venous pursestrings. The venous pursestring is either a single right atrial pursestring in the case of coronary bypass surgery or aortic valve replacement, or in the case of mitral valve replacement or tricuspid valve procedures, a separate superior vena cava and inferior vena cava pursestring. Figure 3.2 shows the cardiac cannulation sites for various procedures as well as the different types of cannulas. After the aortic pursestring has been applied, the patient is heparinized. After the venous pursestring has been placed, the antegrade cardioplegia pursestring is applied followed by the retrograde cardioplegia pursestring. The aorta is cannulated followed by venous can-nulation of either the right atrium alone or the superior and inferior vena cava separately. Then the antegrade plegia line is inserted into the aorta. The retrograde plegia line is inserted through the right atrium directly into the coronary sinus blindly by simply palpating the back of the heart and feeling where the retrograde cannula enters the coronary sinus. Cardiopulmonary bypass is instituted when the activated clotting time is above 400 seconds. In tricuspid valve procedures or anytime the right heart is entered, it is necessary to place caval tapes around the superior and inferior vena cava so that when the right heart is entered air will not be aspirated into the venous cannulae forming an air lock. The tapes
prevent blood from rushing up into the operative field and causing impaired vision.
After cardiopulmonary bypass is instituted, a pursestring is placed in the right superior pulmonary vein for cases of aortic and mitral valve procedures. The different types of cannulas are shown in Figure 3.2. The aortic cannula is shown, as well as the two-stage venous cannula for coronary bypass surgery or for aortic valve replacement. The first stage lies within the right atrium and the second stage lies within the inferior vena cava. A separate superior vena cava and inferior vena cava cannula are shown. The antegrade cardioplegia cannula is shown and actually serves three functions in one. One is to administer cardioplegia; the second is to monitor pressure through a separate line; and the third is to serve as a vent for the aortic root through a separate port. The retrograde cannula for insertion into the coronary sinuses is shown as well and this too has a pressure port as well as a cardioplegia port. The pressure port is very important for the retrograde cannula because overpressurizing the coronary sinus may result in rupture. Pressures should be kept in the range of 20-40 mmHg.
The left atrial/left ventricular vent is shown. The purpose of the vent is 3-fold. First, it prevents overdistension of the heart which would impede coronary perfusion and lead to a very dangerous situation. Second, it evacuates blood which obscures the surgical field. Third, it evacuates air from the left heart during de-airing maneuvers. In the case of aortic valve replacement, this vent actually goes into the left ventricle through the right superior pulmonary vein and left atrium. This differs from mitral valve replacement in which the vent tip lies initially in the left atrium. Only after the mitral procedure has been performed is the left atrial vent pushed visually into the left ventricle.
Methods of venting the left ventricle include the standard vent going through the right superior pulmonary vein through the left atrium, into the left ventricle. Another useful technique involves directly stabbing the apex of the left ventricle and inserting a vent directly through it; this is useful as an emergency procedure. The other way of venting the left ventricle is to place a vent in the pulmonary artery which then aspirates blood retrograde from left heart through the pulmonary veins.
If the heart suddenly distends for no apparent reasons while on bypass and there is no left ventricular vent, one needs to decompress the heart immediately. A distended left ventricle results in inadequate perfusion of the myocardium. One can also decompress the heart by shutting off the pump allowing the venous blood to drain for a few seconds and allowing blood to go from the left ventricle up through the aortic root vent. One can then reduce flow through the aortic line to keep the heart decompressed. If on cardiopulmonary bypass there is an extremely low perfusion pressure in the 20-30 mmHg range despite being on full flow, then one must suspect the possibility of aortic insufficiency and a vent should be placed if not already present via the right superior pulmonary vein into the left atrium and into the left ventricle. If this still does not help and there is still a low perfusion pressure while on cardiopulmonary bypass, the aortic cross-clamp should be applied to prevent regurgitation and the aortic root will then have to be opened in order to administer antegrade cardioplegia directly into the coronary orifices.
To summarize, in coronary artery bypass surgery, one requires an aortic cannula, a two-stage venous cannula, an antegrade cardioplegia cannula, and a retrograde cardioplegia cannula. For aortic valve replacement, one requires an aortic cannula, a two-stage venous cannula, an antegrade plegia cannula, a retrograde plegia cannula, and a left ventricular vent. For mitral valve replacement one requires an aortic cannula, a separate superior vena cava and inferior vena cava cannula, an antegrade plegia cannula, a retrograde plegia cannula, and a left atrial vent. For tricuspid valve replacement, one requires an aortic cannula, separate superior and inferior vena cannulae, antegrade plegia cannula, and caval tapes about the superior and inferior cavae.
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